1. Field of the Invention
This invention relates to devices for detecting faults in cable insulation, and in particular to an electronic device for detecting a fault and providing an alarm signal while laying underground cable.
2. Description of the Prior Art
Telephone and other types of insulating cable are often buried underground by a machine that simultaneously plows a trench and feeds the cable from the reel into the trench. Should the insulation of the cable be cut or punctured by the plow or rocks, it will later have to be dug up and repaired. Normally the cable is not tested for defective insulation until after it has been completely buried. Finding the precise location of the fault in a cable of substantial length can be difficult.
An apparatus is disclosed in U.S. Pat. No. 3,648,282 for detecting insulation faults as the cable is being buried. An electrical potential is attached between the metal shield of the cable and ground. If a fault occurs, the resulting current flowing to ground passes through a relay, which in turn closes contacts to sound an alarm.
The design has inherent disadvantages. A highly sensitive relay is required, since current flow may be small. Relays can be troublesome, especially when used with vibrating heavy equipment operated outdoors. Also, it appears that the sensitivity of the circuit would have to be adjusted for varying earth moisture conditions to assure that a fault would be detected. In addition, the detector is mounted on the tractor, with a collector being used to provide communication between the end of the cable and the circuitry. Changeover to a new reel would be more difficult than with conventional reels because of the collector.